1. Field of the Invention
This invention pertains generally to high-frequency coupling devices, and more particularly to microwave hybrid ring couplers and beam pattern diversity systems utilizing artificial composite right/left-handed transmission lines.
2. Description of Related Art
Couplers are passive microwave components used for distributing or combining microwave signals. Couplers are usually three or four-port devices used for injecting a second signal into a network, or as a means to sample a signal within a network, while these couplers also provide reciprocal functions. Couplers are used in circuits to generate separate signal channels with desirable characteristics. Conventional couplers may be divided into two categories: coupled-line couplers (backward, forward) and tight-couplers (e.g., branch-line, hybrid ring, and so forth). While the former are limited to loose coupling levels (typically less than −3 dB) because of the excessively small gap required for tight coupling, the latter are limited in bandwidth (i.e., typically less than 20%).
Conventional hybrid ring couplers, also referred to as 3 dB, 180° hybrid ring couplers, are often referred to as rat-race couplers in view of their circular shape as shown in FIG. 1. The hybrid ring coupler 10 is a ring-shaped transmission line 12 having four ports for equally splitting an input signal or for generating a sum or difference in the signals. Ports are shown in FIG. 1 as a summation port (Port 1) 14, a first output port (Port 2) 16, a second output port (Port 3) 18, and a difference port (Port 4) 20. One of the benefits of using a hybrid ring is that it alternately provides equally-split, but 180 degree phase-shifted, output signals. It should be appreciated that the coupler may be similarly utilized for input as well. In a conventional hybrid ring coupler the center conductor ring is 1½ wavelengths in circumference (or six ¼ wavelengths) and each port is separated by 90°. This configuration creates a loss-less device with low variable standing wave ratio (VSWR), excellent phase and amplitude balance, high output isolation and matching output impedances. Yet these rat-race (hybrid ring) couplers have the shortcomings of narrow bandwidth and large size. Applications for rat-race couplers are numerous, including mixers and phase shifters, and so forth.
However, the use of couplers is often problematic as many wireless LAN systems operate in two frequency bands having some desired relationship, and thus require dual-band components, such as the use of two hybrid ring couplers. Furthermore, the need for small and light-weight systems lead to the desire to employ compact components in front-end systems. Conventional couplers exhibit an inherent harmonic relationship between their operating frequencies, while the operating frequency and the physical dimensions of the structure make it challenging to design a compact dual-band component based on conventional methodology.
Due to the requirement for increasing levels of data throughput on limited channels, researchers in the wireless communication field have substantially directed their efforts toward increasing capacity without occupying more spectral resources. Multiple-input multiple-output (MIMO) systems have received recent attention in wireless communications because of their attractive capability of linearly increasing capacity with respect to the number of antennas in the transmitter/receiver space. Derived from the MIMO concept, space, polarization, and pattern diversity are three common approaches to enhance the channel capacity. Among these, pattern diversity is preferred for a number of applications as it has low space requirements and presents a set of orthogonal radiation patterns using a mode decoupling network. In certain applications, conventional 90° hybrids have been used to achieve pattern orthogonality generated by exciting either of the input ports. Although some attention has been focused on antenna pattern diversity systems, there has been scant attention focused on dual-band applications and thus system applicability is restricted. It should be appreciated that conventional couplers can be used as dual-band components only at odd multiple frequencies of the first band.
Accordingly, a need exists for a system and method of coupling microwave signals while not being constrained to operating frequency relationships or a single relationship between operating frequency and physical dimensions. These needs and others are met within the present invention, which overcomes the deficiencies of previously developed coupler system and methods.